Retroviral insertion occurs just upstream of the first coding exon. PCR analysis on whole brain samples from one-day old (P1) WT, heterozygous and homozygous littermates showed approximately 70% and nearly 100% reduction of mRNA expression from this allele in the brains of heterozygous and homozygous mutants, respectively. Western blotting indicated that protein expression in the brains of P1 heterozygous and homozygous mutants from this allele, compared to WT, was reduced by approximately 35% (p < 0.05) and 60% (p < 0.001), respectively, suggesting that the gene trap mutation is hypomorphic. (J:103485, J:250111)